1. Field of the Invention
This invention relates to an electric hoist having overload safety means for preventing lifting, lowering and dragging under overload condition and capable of securely lowering a load irrespective of its light or heavy weight.
2. Description of the Prior Art
With electric hoists such as electric chain blocks whose load sheave is driven through reduction gears by electric motors, it has been proposed to provide on a driven shaft of a reduction gear mechanism a brake assembly consisting of a brake support member, a brake retainer member, a brake ratchet wheel and friction plates between the members and further provide an overload safety device including frictional connections interposed between the brake retainer member and a driven gear fitted thereon, in order to obstruct raising of a load by slippage in the overload safety device when the load sheave is subjected to an overload.
With such an electric hoist, however, the friction transmission force of the overload safety device often becomes less than that of the brake assembly owing to its overtightening, so that once raised a load cannot be lowered due to slippage in the overload safety device.
In order to solve this problem, it has been proposed to interpose a one-way clutch between the driven gear and the brake retainer member to render inoperative the overload safety device when lowering the load, as disclosed for example in Japanese Patent Application Publication No. 42,937/78. In this case, however, as the overload cannot be prevented in lowering the load, it is indispensably required to provide a limit switch for detecting a lowermost position of the load in view of the safety in operation which would possibly make the hoist complicated.